gistfile1.rb

class Account < ActiveRecord::Base
 has_may :piece_of_works

 # some code
end

class Customer < Account
 has_many :projects
end

class Supplier < Account
  has_many :jobs
end

class PieceOfWork < ActiveRecord::Base
  belongs_to :account
end

class Project < PieceOfWork
  belongs_to :customer
end

class Job < PieceOfWork
  belongs_to :supplier
end